The present invention relates to a radio transmitter/receiver (referred to as "transceiver", hereinafter) having an unlock alarm of a frequency synthesizer including a PLL (phase locked loop) as one of the device alarms and, particularly, to such a transceiver suitable for use in a radio communication system such as a time division multiple access (TDMA) communications system which requires high-speed and frequent switching between a receiving state and a transmitting state.